1. Field of the Invention
The present invention relates to a wax transfer type thermal printing method and a thermal printer, and more particularly to a method and a printer suitable for printing a composite image including characters and a half-tone image.
2. Related Art
In a wax transfer type thermal printing method, a back surface of an ink film is heated with a thermal head, and melted or softened ink is transferred to a receiving paper. Because the amount of ink to be transferred to the receiving paper cannot be adjusted in accordance with heat energy, this method has been used to print bimodal images such as a line drawing or characters. Japanese Patent Laid-open Publication No. 3-219969 discloses an improved wax transfer type thermal printing method capable of printing a half-tone image having a high gradation by using a plurality of heating elements or resistive elements which are long in a main scanning direction and narrow in a sub scanning direction perpendicular to the main scanning direction. Thereby, the length of a recorded ink dot in the sub scanning direction may be changed by control of the time of current supply, the amount of current or the number of drive pulses for the heating elements.
Characters, half-tone images and composite images including a half-tone image and characters as printing images may be used for thermal printing. Generally, because high resolution is required to print characters, it is necessary to determine a pixel density for characters to be 12 dots/mm or more. In this case, the size of a pixel is 84.times.84 .mu.m or smaller. On the other hand, a pixel density of 6 dots/mm is sufficient to reproduce most half-tone images. Accordingly, a thermal head with a different pixel density is used for a wax transfer type thermal printer for characters and a wax transfer type thermal printer for half-tone images.
However, in a wax transfer type thermal printer for both characters and half-tone images, a thermal head of 12 dots/mm or more in the pixel density is used so as to adapt to the pixel density for characters for recording an ink dot in each pixel of e.g., 84.times.84 .mu.m or smaller. In this cased the following problems occur because the size of pixels is too small to print half-tone images at this pixel density. First, ink dots cannot be transferred uniformly at low density areas because of irregularities of a receiving material such as plain paper so that the density of the areas will not be even. Second, when the length of heating elements in the sub scan direction is made constant for both characters and half-tone images, a sufficient number of gradations for half-tone images cannot be obtained. Third, the occurrence of fogging would increase.